468 
Dr. A. L. Thomson : Results of a Study of [Ibis, 
height in 1914, and that it would have continued at a high 
level lor a few years i£ circumstances had remained normal. 
As it was, the work not only entered on a premature decline, 
but a great deal of it was probably rendered less fruitful by 
unfavourable conditions for the reporting of reappearances. 
The Purposes of Bird-Marking. 
In its essentials the method consists of the marking of a 
large number of birds, in some way or other, for the sake of 
the data afforded by the subsequent reappearance of a small 
proportion of them. The principle is identical with that of 
the well-known method of marking fishes as a means of 
studying their movements and life-histories, while an analo¬ 
gous system has also been utilised in the study of ocean 
currents. 
The method differs from other methods of studying migra¬ 
tion in that it approaches the problems from the individual 
aspect—it begins with individual birds and works from them 
towards general movements. A marking record implies 
that there are two or more times in the life of a particular 
bird when one is able to state with certainty its whereabouts 
and various other facts, these times being usually those of 
infancy and of death. When a large number of these 
records has been collected and correlated, there will exist 
an array of facts which could not have been ascertained by 
other means. 
To estimate the value of information of this kind, the nature 
of the problems must first be considered. Perhaps the 
greatest and most difficult problem of migration is that of its 
origin—its ultimate cause. To an appreciable extent the 
purpose served by migration, its raison d’etre, is known, 
and the immediate factors which periodically stimulate the 
migrational habit into being may be surmised : but the ques¬ 
tion of the origin of the habit still lies completely within the 
realm of conflicting theory. A matter of theory and hypo¬ 
thesis it must doubtless ever remain, but one may at least 
put the theories to the test of facts and eliminate those that 
are found wanting. One thing seems obvious, and that is 
